technical How to Design and Install Bevel Gears for Optimum Performance: Lessons Learned Stephen Marsh Introduction the assembly. This is the most important parameter for ensuring Bevel gears must be assembled in a specific way to ensure correct and optimal operation. smooth running and optimum load distribution between gears. Typically, the pitch cone apex-to-crown-value is a fixed, nom- While it is certainly true that the “setting” or “laying out” of a inal value determined by a formula. The dimension between pair of bevel gears is more complicated than laying out a pair the mounting surface (in Figure 2 it is the bearing seat) and the of spur gears, it is also true that following the correct procedure crown point will have a tolerance; it is common to see a toler- can make the task much easier. You cannot install bevel gears in ance of ± 0.05 mm up to ± 0.1 mm. Obviously the mounting dis- the same manner as spur and helical gears and expect them to tance is the addition of the two values and will therefore also behave and perform as well; to optimize the performance of any have a tolerance. two bevel gears, the gears must be positioned together so that For straight bevel gears, the pitch cone apex is the intersection they run smoothly without binding and/or excessive backlash. point of the pinion and gear axes (Fig. 2). Bevel gears can include straight, spiral, Zerol, hypoid and The pitch cone apex-to-crown-value is provided in engineer- Spiroid (to address the differences between each one is beyond ing handbooks and in publications provided by Gleason Corp., the scope of this guide). Because these types of bevel gears are for example. Gleason publications include most of the informa- basically conical in shape, they all have an optimum position for tion required by the gear designer to select a satisfactory pair of best performance. Usually the manufacturer of the bevel gear bevel gears. The pitch cone apex-to-crown value is determined determines this optimum position by running tests of individual by a formula that is a function of the pitch diameter, pitch angle gear sets. However, the gearbox designer and assembly technician and addendum; therefore, the pitch cone apex-to-crown-value share responsibility for incorporating this optimum positioning cannot be chosen arbitrarily. However, the mounting distance in the gearbox. The designer must provide shimming dimen- is determined by the designer as long as the pitch cone apex-to- sions that are easy to measure in order to aid the fitter/techni- crown-value is maintained. (Of course, in reality this mounting cian in assembling the overall gearbox. The aim of this guide is distance will have to have some degree of tolerance.) It may be to provide instruction on how best to accomplish this. possible to manufacture all gears to the nominal mounting dis- tance specified on the drawing, but the additional cost to do so Part I: Key Factors is usually not warranted. Various parameters contribute to proper gearbox assembly If two gears are being machined as a set, the manufacturer that help ensure smooth and efficient operation; the two most can establish the optimum value for this distance by running the important criteria are: gear set and adjusting its position to obtain a tooth contact pat- 1. Mounting distance tern that is consistent with smooth running and optimum load 2. Backlash distribution between mating gear teeth. The optimum mounting Mounting distance. The distance from a locating surface on distance can then be recorded. the back of one gear (most commonly a bearing seat) to the Because of dimensional variations between parts, each gear center-line of a mating gear is the mounting distance (Fig. 1). will have a unique value for the mounting distance and, in most In some cases, for convenience, a front surface may be used for cases, the manufacturer permanently marks this value onto each gear; the mounting distance will be within the tolerance Figure 2 Assembly of a pair of straight bevel gears (pinion and gear). Figure 1 Gear showing make-up of the mounting distance. [www.geartechnology.com] 60 GEAR TECHNOLOGY | June/July 2013 specified on the drawing. Manufacturers aim for a tolerance of It is good practice for the designer to make provision so that ± 0.05 mm from the nominal value. If, at assembly, the mounting the teeth can be inspected and that assembly personnel can also distance of one or both gears is made at less than the dimension check for backlash, etc. This feature can also be used for inspec- specified, the teeth may bind, and excessive wear or breakage tion during service to check the condition of the teeth without can result. If the mounting distance of either gear is made longer disassembling the gearbox. Once the components are machined, than the dimension specified, the gears will not be in full mesh they must be fully inspected to check the housing and mount- on a common pitch line and may have excessive backlash. ing components for errors. While the shaft angles, offset and Backlash. The second most important variable for a pair of mounting distances are the usual items to check, the alignment bevel gears is backlash—i.e., the space between mating gear of the bores and the squareness of the bearing seats are also teeth or the difference in width of the gear tooth and pinion important. tooth of the mating gear. Unless otherwise specified, it is mea- Bearings. While bearings are not a focus of this guide, it must sured at the tightest point of the mesh. be noted that the bearings of a gear system are responsible for Backlash is necessary to achieve correct operation of the holding the components in position, absorbing and distribut- gears and varies with the size of the tooth and operating condi- ing forces and moments, and assuring the rotation of shafts and tions. Bevel gears are cut to have a definite amount of backlash gears. Any clearance in the radial and axial directions inside the when correctly assembled together. But excessive backlash or bearing can be adjusted by axial pre-loading of the inner or outer play, if great enough, can cause a sudden impulse or shock load ring or race. This pre-loads the rolling bodies and bushings and in starting or reversing that may cause serious tooth damage. achieves a stiff, play-free radial and axial support bearing. Excessive or insufficient backlash can also result in noise, exces- Ideally, the bearing manufacturer should be consulted regard- sive wear and damage. Backlash can be changed by changing the ing the selection of bearings suitable for the radial and thrust position of one or both gears. loading required to maintain the alignment and position of the bearing and pinion. The axial and radial movement of the bear- General Notes for the Design Engineer ings should not allow the load to substantially shift, and the Gearbox housing/mounting. It is very important that the bearings should be stiff enough to prevent deflection in order to designer consider the mountings for the gear and pinion so maintain proper contact. Deflection of the bearing shaft and the that they are rigidly supported to handle all loads to which the housing are all influenced by bearing contact. It is highly recom- gears will be subjected in service. It is equally important that the mended that the bearings be pre-loaded; this should be man- designer detail the housing or mounting parts (e.g., bearing cap) datory—even in cases of small bevel gear sets with low torque with adequate tolerance for alignment, squareness, fits, and run- transmission. out, etc. While bevel and hypoid gears can accommodate rea- Solid pre-load is obtained by mechanically locking the races sonable displacements and misalignments without detriment to of the bearing into position with the aid of shims. The shims tooth action, excessive misalignment of the gears reduces their and, hence, bearing races, spacers, etc., are subjected to an axial load capability, with the consequent danger of surface failure load; this axial load is commonly provided by the end-caps. By and breakage while in service. adjusting the thickness of the shims, the amount of pre-load Provision should be made by the designer in the design of the can be increased or decreased. This must be taken into account housing and mounting parts so that both the pinion and gear when determining the range of shims during the design stage mounting distances can be adjusted. This is usually done using and determining the actual size of the required shim at assem- shims. Determining the range of shim thicknesses can be cal- bly. The disadvantage of the solid pre-load method, however, culated by the designer so that the mounting distances of the is the high variation in pre-load as temperature changes, and gears are always maintained. An example of setting up gears and its reduction as the bearing wears. Pre-loads can be applied by determining the range of shims is explained in more depth later a spring to counteract this and provide a consistent pre-load as in this guide. temperature varies, but generally this solution is more complex and is beyond the scope of this guide. It must be remembered that excessive pre-load will reduce the life of the bearing and increase the amount of noise produced and heat generated, as well as the bearing starting and running torque. The amount of pre-load required is determined by vir- tue of design and stress analysis.